PLUGS FOUWLING AFTER SOME RUN TIME 1998 40hp mariner

[BMT4387]

I have a 1998 40hp Mariner till steer.(40ELHPTO) When take the boat out it will start right up and run great. After some time going from full speed down to an idle and back up. The motor begins to bog down and sometimes will not go back up to full speed. I have found that one of the plugs will be wet when I pull it out. What could be causing this? And what could I do to solve this problem? Thank you.